Nucleate boiling is the familiar bubbling process which may be so gentle that only small bubbles are produced or extremely vigorous if the fluid interface temperature is sufficiently high. Such nucleation within a liquid in contact with a solid heating surface occurs at minute cavities or other irregularities in the surface. If the coolant has a high tendency to wet the surface such as the non-aqueous coolants discussed in my U.S. Pat. No. 5,031,579, the shape of the bubble is pinched in at the metal surface and readily detaches itself. If, on the other hand, the coolant has a low tendency to wet the surface such as an aqueous engine coolant, for example, a 50/50 water ethylene glycol solution discussed in my co-pending application Ser. No. 907,392, filed Jul. 1, 1992, the bubble grows at the surface and is set free only when it is comparatively large. Experiments have shown that when the temperature of the heating surface is first raised above that of the surrounding bulk liquid, most of the temperature drop takes place across the very thin layer of liquid adjacent to the surface. As the temperature difference is increased the thickness of the layer also increases at a rate approximately proportional to the increase in temperature differential. This state of affairs does not continue indefinitely, however, since the rate of increase of thickness decreases and the layer reaches its maximum when bubbles form. The vapor bubble promotes turbulence as well as being a carrier of latent heat of vaporization. Bubbles formed on the surface in this superheated layer force back the liquid immediately surrounding them and, on breaking free from the surface, the surrounding liquid is caused to flow to the space previously occupied by the bubbles. The rapid growth and departure of many bubbles, and the resulting source and wake flows in the liquid, cause large oscillations in the superheated film. It is generally accepted that the major portion of the heat for bubble growth is transferred from the heating surface to the bubble by the superheated liquid layer through a conduction or convection process. The growth and departure of the bubble breaks down the superheated film and brings cool liquid to the heating surface. It is also to be noted that, as indicative from testing and in numerous technical references, increasing the coolant velocity reduces the metal temperature in the convection region for a given heat flux and also suppresses nucleate boiling.
In order to achieve peak efficiency of coolant flow in the non-aqueous cooling system taught in my U.S. Pat. No. 5,031,579 and for the aqueous reverse-flow system taught in my co-pending application Ser. No. 907,392, it is desirable to control the volume of vapor, or in other words, nucleate boiling generated in the head chamber. Additionally, it is desirable, when employing a reverse-flow coolant direction, to offset the dynamic loss exhibited in conventional systems wherein upward motion of the coolant assists the natural buoyancy of the coolant vapor to release from the critical metal surfaces in the head cooling chamber over the area of the combustion chamber domes. The dynamic's of coolant vapor resistance to release from the metal surface of the cooling jacket is a major defect of known aqueous reverse-flow cooling systems.
Accordingly, cooling flow rate through the head cooling chamber must be established to create turbulence on the metal surfaces, particularly the surfaces over the combustion domes. When the proper flow rate is established three major improvements occur all of which tend to reduce the volume of vapor generated in the head chamber.
(1) As shown by testing, the metal temperature at any given heat flux will be reduced and nucleate boiling will be suppressed due to a reduction in vapor points of origin.
(2) The total heat exchange value will be of a higher magnitude for any given load or heat flux because of the increase in "bulk" heat exchange from the metal to the coolant. The metal will stay under control evidencing a longer rise time to the nucleate boil point.
(3) In reverse flow systems, turbulence and coolant scrubbing of vapor off metal surfaces increases with the flow of the coolant, compensating for the dynamic directional flow lost as exhibited in conventional upward flow systems. Coolant turbulence dictated by higher flow velocities not only breaks away vapor on the hot jacket surfaces over the combustion domes, but by breaking away, the vapor allows improved "wetting" of the surface. "Wetting" of the surface increases contact of the coolant at critical hot spots and effects a reduction of nucleate boiling and a reduction of vapor generations.
The efficiency of the pump is a factor in establishing the proper flow for the non-aqueous system taught in my U.S. Pat. No. 5,031,579 as well as in the aqueous reverse-flow cooling system taught in my co-pending application Ser. No. 907,392. It is to be noted that many pumps currently used in production vehicles which may appear to produce insufficient flow, become usable if the other components of the system are maximized for proper flow. One such important component is the thermostat.